TlBa_1.2La_0.8CuO₅: A superconductor produced by reduction, rather than oxidation, of the CuO₂ planes

Doping of La³⁺ for Ba²⁺ in TlBa₂CuO₅, to introduce holes into the CuO₂ planes, leads to superconductivity in the material TlBa_1.2La_0.8CuO₅ (with T_c ≈ 52 K). The structure (P4/mmm) has been refined using neutron powder diffraction data collected at 305 K (a = 3.8479 (3) A ̊ and c = 9.0909 (6) A ̊) and 15 K (a = 3.8444 (1) A ̊ and c = 9.0744 (3) A ̊). An increase in the Cu-O in-plane bond distance from that in TlBa₂CuO₅ is consistent with a lowering of the formal oxidation state of copper from its value of 3+. Substantial static site disorder is observed in the Tl-O (3) planes, presumably to allow for better Tl³⁺-O coordination. Examination of the thermal parameters for the CuO₆ octahedra shows that this disorder in turn produces a local tilting of the octahedra and a resultant puckering of the CuO₂ planes.